Move Doxygen deploy location to GitHub Actions

[wysiwyng]

GitHub Actions has gained a feature where GitHub Pages hosted content can be deployed directly from an Actions workflow, no separate branch storing documentation data needed. See:

https://docs.github.com/en/pages/getting-started-with-github-pages/configuring-a-publishing-source-for-your-github-pages-site#publishing-with-a-custom-github-actions-workflow
https://github.blog/changelog/2022-07-27-github-pages-custom-github-actions-workflows-beta/

This fixes #83. If we roll out this change, we should also think about removing the gh-pages branch along with all commits related to it to reduce the repository size.

[rafzi]

we should also think about removing the gh-pages branch along with all commits related to it to reduce the repository size.

done, but it should have always just been one commit that is force pushed. any unreferenced commits should be pruned by github regularily.